Scientific Method —

Our complicated past

Comparing the human and chimp genomes has led some researchers to a startling …

The fossil evidence for the divergence of the hominid lineage (which led to us) from the one that led to chimps seemed fairly clear. By 6.5 million years ago, fossils from Chad showed that animals with clearly hominid features existed, and the molecular data largely agreed that the divergence occurred in that time frame. But the completion of both the human and chimp genomes have opened the topic up to studies on a completely different scale, and a recent analysis is suggesting that the picture is not as clear as it seemed.

On average, the two genomes agree on a date in the 6 million year range, but a new analysis to be published in Nature looked at the dates when individual chromosomes within the genome diverged. The analysis places the timing of divergence at 6.3 million years, well after the time of the Chad fossil. The big surprise, however, is that some regions of the genome split substantially later than that: the X chromosome, for example, appears to be the most recent in origin, and dates from over a million years later. Overall, the range of divergence dates covers nearly 4 million years, depending on where in the genomes you do the comparison.

The only thing that makes sense out of this mix of dates is that the hominid and chimp lineages interbred several times after they separated. As the X chromosome contains many fertility genes, this could explain why it is the most recently diverged, as it may have been involved in ensuring the fertility of the hybrid offspring. Two things are important to point out here. One is that the interbreeding is not as dramatic as it sounds, as we would probably consider the hominids of 5 million years ago to be pretty ape-like in most ways. The second is that this shouldn't be that surprising; we just covered how two apparently different mammalian species (polar and grizzly bears in that case) separated by a million years of evolution could still produce a fertile hybrid.

The Nature story claims there may be a need to potentially revise our understanding of the Chad fossils, but that seems to be a bit of an overreaction to me. If the initial analysis of the fossil was correct, it does almost certainly represent a precursor to the hominid lineage, and it may very well have existed as part of a species that was largely reproductively isolated from the chimp lineage. But just as clearly, there were exceptions to any such isolation. Our genomes tell us that the the two lineages continued to exchange genetic material long after they started to go their separate ways, but those exchanges might very well have been rare exceptions.